Transcript Hurricanes

Hurricanes
Earth’s Largest Storms
Hurricanes
• Hurricanes are the largest storms
that occur on Earth.
• Hurricanes are very large, swirling,
low pressure systems that form
over tropical waters (water temp.
must be at least 800 F).
• Hurricanes can last for days due to
the constant supply of energy from
the warm waters.
When Does A Storm Become
A Hurricane?
• To be classified as a hurricane
there must be sustained winds of
greater than 74mph.
– Greatest winds recorded are
236mph; Category 5 “Mitch” in
Nov 1998= most deadly
hurricane ever.
What is a Pressure System?
• Pressure = as particles move
and collide they exert
pressure.
• Differences in air pressure
have a great affect on the
weather.
Pressure Systems
• When particles are more densely
packed together they exert high
pressure.
– High air pressure usually means clear,
fair weather.
• When they are less densely packed
they exert less (low) pressure.
– Low air pressure usually means clouds and
rainy weather.
High Pressure System
• Cold air is dense and it sinks.
• As the air sinks it warms up.
• Warm air holds more water vapor.
– The total amount of water vapor remains
the same.
• As the air warms the relative humidity
decreases as droplets in the clouds
evaporate.
• A high pressure system usually means fair
weather as the moisture in the air is
evaporated so few clouds form.
Low Pressure System
• Warm air has low density and it rises (forced
upwards by surrounding denser air).
• As the air rises it cools.
• As the air cools the relative humidity increases –
eventually reaching the dew point.
– At the dew point condensation takes place and
clouds form.
• Low pressure systems usually form along fronts
where warm air and cold air meet.
• A low pressure system leads to precipitation in the
form of rain, snow, hail, sleet.
• Low pressure systems cause most of the severe
weather in the US.
Where Hurricanes Form
• Hurricanes form in the “Tropics”.
• The “Tropics” are centered around
the equator (00 latitude) extending
north to the Tropic of Cancer (23.50
N latitude) and south to the Tropic of
Capricorn (23.50 S latitude).
Hurricane Origins
Pinpointing Locations On Earth
• In order to find exact locations on
Earth a system of grid lines is used.
(Similar to graph paper).
• Lines of latitude run east/west around
the earth, parallel to the equator.
• Lines of longitude run north/south
through the poles of the earth.
Latitude
• The latitude lines are east/west lines based
on the geometry of the earth using the axis
of the earth and the equator.
– Latitude range is 00 to 900.
• The equator is 00 latitude, (this is the
longest distance around the earth traveling on
the same latitude line).
• The North Pole is 900 North latitude.
• The South Pole is 900 South latitude.
• Your latitude is the angle created using the
axis (center of the earth), the equator and
your location.
Latitude Lines
Longitude
• The longitude lines are north/south lines,
based on time (the rotation of the earth),
using the Prime Meridian and the North and
South Poles. Longitude range is 00 to 1800.
• Longitude lines are all the same length as
each one connects the North and South Poles
and splits the earth into equal halves – this is
why they are often referred to as meridians.
• The Prime Meridian is 00 longitude.
• The Prime Meridian passes through
Greenwich, England which is the time keeping
capital of the world.
• Your longitude is your time (of the Earth’s
rotation) away from the Prime Meridian.
Earth’s Lines of Longitude
How Hurricanes Form
• Hurricanes form over warm tropical oceans where
two opposing winds meet and begin to swirl.
• A low pressure forms in the middle of the swirling
winds and begins rotating.
• Warm moist air is forced upwards in the center.
• The dropping pressure in the center pulls more air
toward the center creating increasing winds and
lower pressure. This cycle of increasing strength
continues as long as the storm remains over warm
water.
• Hurricanes weaken as they hit land because there is
no longer a supply of energy (from the warm water)
available.
Life Cycle of a Hurricane
1. Form first as a low pressure system,
2. Grow into a tropical depression
(<31mph winds),
3. Grow into a tropical storm
(32-74mph winds),
4. Grow into a hurricane
(>74mph winds).
• Out of the 10 tropical storms per
year average, 6 will develop into
hurricanes and 2 will strike the US.
Measuring Strength of
Hurricanes
• The strength of hurricanes is measured
using the Saffir-Simpson Scale. This
scale takes into account several
variables of hurricanes including
atmospheric pressure, wind speed and
storm surge.
• Once a storm becomes a hurricane they
can range from Category 1 (weakest)
to Category 5 (strongest).
Hurricane Strength
The Saffir – Simpson Scale
Category
1
Atm.
Wind Speed
Pressure
(mph)
(mb.)
>980
74 – 95
Storm
Surge
(feet)
4–5
2
965 -979
96 – 110
6–8
3
945 -964
111 – 130
9 – 12
4
920 -944
131 – 155
13 – 18
5
<920
>155
>18